1. FIELD OF THE INVENTION
The present invention relates to a Hall effect type sensing device which can be used, for example, for detecting a crank angle in a distributor for internal combustion engine ignition systems.
2. DISCUSSION OF BACKGROUND
Referring to FIG. 3, there is shown a partial cross sectional view of an example of a conventional internal combustion engine ignition distributor. Reference numeral 1 designates a distributor housing. Reference numeral 2 designates a distributor cap which is used to cover an opening in the housing 1. Reference numeral 3 designates a distributor shaft which is rotatably supported by the housing 1, and which is rotated in synchronism with an crank shaft (not shown) of an internal combustion engine. Reference numeral 4 designates a Hall effect type sensing device which is fixed within the housing 1. Reference numeral 5 designates a vane which can be carried on the distributor shaft 3 to rotate therewith as one unit, which can be made of magnetic material to work as a magnetic flux shutter, and which is formed in a circular shape and has the periphery formed as downwardly extending bent portions 5a, the bent portions being equally spaced at 90.degree. etc. apart. The bent portions 5a can be formed to pass through an air gap 6 which is formed in the Hall effect type sensing device 4.
Reference numeral 7 designates a distributor rotor which is fixed to the leading end of the distributor shaft 3 to rotate therewith as one unit, and which carries a rotor electrode 9. The electrode 9 is in sliding contact with a central electrode 8 which is arranged in the cap 2. Reference numeral 10 designates peripheral electrodes which are arranged within the cap 2 to correspond to ignition plugs (not shown) of the cylinders of an internal combustion engine (not shown). The secondary voltage of an ignition coil (not shown) is supplied to the peripheral electrodes 10 through the rotor electrode 9 according to the firing order, thereby sparking the ignition plugs.
Referring now to FIG. 4, there is shown an enlarged cross sectional view of the Hall effect type sensing device 4 shown in FIG. 3. Reference numeral 20 designates a casing which is made of thermoplastic resin. Reference numeral 21a designates a first magnetic flux guide which is in the form of L character in section, and which is housed in the casing 20. Reference numeral 22 designates a permanent magnet which is bonded to the first magnetic flux guide 21a. Reference numeral 23 designates a ceramic board which is housed in the casing 20. Reference numeral 24 designates a Hall transducer which is mounted on the ceramic board 23. Reference numeral 21b designates a second magnetic flux guide which is in the form of L character in section, which is bonded to the Hall transducer 24, and which is arranged to be opposite to the first magnetic flux guide 21a.
In the internal combustion ignition distributor having such structure, when the distributor shaft 3 is rotated by the crank shaft of the internal combustion engine, the vane 5 which is fixed to the distributor shaft 3 as one unit is also rotated. Each time the respective bent portions 5a have passed through the air gap 6, the bent portions 5a shunt the magnetic flux which extends from the magnet 22 toward the Hall transducer 24. In other words, when one of the bent portions 5a which are made of magnetic material is passing through the air gap 6, the magnetic flux from the magnet 22 is directed to the first magnetic flux guide 21a through that bent portion 5a to prevent the magnetic flux from through the Hall transducer 24. As a result, a change in the magnetic flux is given to the Hall transducer 24, depending on the revolution of the distributor shaft 3. The Hall transducer 24 converts the change in the magnetic flux into an electric signal, which is in turn transmitted to an ignition timing control unit (not shown) to control the primary current of the ignition coil, thereby causing the secondary voltage to generate in the ignition coil at every ignition timing. The secondary voltage is fed through the central electrode 8 and the rotor electrode 9 to the peripheral electrodes 10 in the firing order, depending on the distributor rotor 7. In this way, the cylinder ignition plugs are fired in the firing order, allowing the internal combustion engine to be continuously driven.
In the Hall effect type sensing device 4, distance variations between the magnet 22 and the hall transducer 24 have significant effects on crank shaft angle detection accuracy. Although forcibly positioning the magnet 22 to the casing 20 is allowed by e.g. fit, positioning the Hall transducer 24 can not be made in a similar manner because it is an electronic part. When a Hall IC assembly which is constituted by the ceramic board 23, the Hall transducer 24 and the second magnetic flux guide 21b is press-fitted into the casing for positioning, there is a possibility that the ceramic board 23 could be damaged, or that a change in properties of the Hall transducer 24 could be caused due to distortion of the Hall transducer. In order to eliminate such possibility, the Hall IC assembly has to be mounted into the casing 20 to have a clearance 25 between itself and the wall of the casing 20.
The conventional hall effect type sensing device 4 constructed as stated earlier has such an arrangement that the Hall IC assembly is mounted in the casing 20 to have the clearance 25 between itself and the wall of the casing 20. This arrangement creates a problem in that a variation in distance between the magnet 22 and the Hall transducer 24 produces a change in the magnetic flux from the magnet 22 to the Hall transducer 24 to have an adverse effect upon a crank shaft angle detection accuracy.